1. Field of the Invention
This invention relates to a porous material of an oxide and/or a complex oxide mainly comprising alumina, zirconia, titania, magnesia, iron oxide or ceria, a process of producing the porous material, a catalyst for purifying exhaust gases comprising the porous material, and a method of purifying exhaust gases.
More specifically, it relates to a porous material having a spongy structure which is suitable for use as a catalyst, a carrier for catalysts, various fillers, a carrier for enzymes, an adsorbent, a filler, and so forth and which is characterized in that (1) the mean pore diameter is in a meson-pore region, (2) the pores have a sharp size distribution, (3) at least a part of the pores form a three-dimensional network structure, and (4) the porous material has substantially no fibrous structure, and a porous material having the above characteristics (1) to (4) which is made up of particles having an aspect ratio of 3 or smaller aggregated together while leaving pores among them; and a process for producing these porous materials.
The present invention also relates to a catalyst and a method for purifying exhaust gases from internal combustion engines of automobiles and the like. More specifically, it relates to a three-way catalyst used for engines run around a stoichiometric air/fuel ratio and a catalyst used for so-called lean-burn engines operated in an oxygen-excess atmosphere. Still more specifically, the invention relates to a three-way catalyst for purifying exhaust gases from conventional engines through simultaneous reduction/oxidation of carbon monoxide (CO), hydrogen (H2), hydrocarbons (HC), and nitrogen oxides (NOx), a catalyst for efficiently reducing nitrogen oxides (NOx) in oxygen-excess exhaust gases which contain oxygen in excess of the amount required to completely oxidize the reducing components, such as carbon monoxide (CO), hydrogen (H2), and hydrocarbons (HC), and a method for purifying exhaust gases.
2. Description of Related Art
The present invention covers the field of a porous material and the field of exhaust gas purification. Disadvantages or drawbacks of related arts are described below separately.
With respect to an alumina porous material having an appropriate pore structure, JP-A-58-190823 and JP-A-60-54917 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) disclose alumina carriers which have a large pore size with a narrow pore size distribution and yet exhibit excellent mechanical strength.
JP-A-55-27830 teaches a process for producing an alumina porous material having the above-described pore structure, which comprises repeating the steps of adjusting the pH of an aluminum hydroxide slurry to 5 or lower, or 11 or higher and then adjusting the pH to 6 to 10 by addition of a neutralizing agent (a pH swing method). Analogous methods are disclosed in JP-A-58-190823 and JP-A-60-54917.
In regards to a silica porous material having an appropriate pore structure, JP-B-7-64543 (the term “JP-B” as used herein means an “examined Japanese patent publication”) discloses spherical silica particles having a pore volume of 0.8 to 1.8 ml/g, a surface area of 20 to 500 m2/g, and an average pore size of 80 to 1000 Å. It also teaches a process for preparing the silica porous material comprising drying silica hydrogel, obtained by neutralization of an aqueous alkali silicate solution, with superheated steam at 100 to 1000° C. to give silica xerogel. According to the disclosure, it is preferred that the silica hydrogen be previously aged under 0.5 to 5 kg/cm2 of steam for 0.5 to 24 hours.
As for a zirconia porous material, JP-A-8-66631 discloses porous zirconia particles having a sharp pore size distribution that is an important character for use in liquid chromatography, which are obtained by incorporating 0.1 to 10% by weight of silica into zirconium oxide so that the crystal form of zirconium oxide may be prevented from changing during calcination.
With reference to a titania porous material, JP-A-6-340421 proposes needle-like, porous, and fine titanium oxide particles having an average breadth of 80 to 120 Å, an average length of 240 to 500 Å, and an aspect ratio of 2.4 to 6.4, which is produced by a process comprising the steps of (a) allowing a hydrolyzable titanium oxide compound to react with a base to precipitate hydrated titanium oxide, (b) adding a polybasic carboxylic acid to the reaction system to dissolve the hydrated titanium oxide, (c) adding an alkali to the reaction system to hydrolyze the chelated titanium compound, (d) adding an inorganic acid to the precipitate and stirring the system to deflocculate, and (e) dehydrating the resulting fine particles and calcining at 200 to 400° C.
Concerning a magnesia porous material, JP-A-59-232915 discloses a process for producing spinel comprising adjusting the pH of a mixed aqueous solution of a water-soluble magnesium salt and a water-soluble aluminum salt with an alkali in the presence of an alcohol to form a precipitate and drying and calcining the precipitate.
As regards an iron oxide porous material, JP-A-61-268358 describes an iron oxide porous material comprising iron oxide and chromium oxide, having a large pore size with a narrow pore size distribution, and exhibiting excellent durability against oxidation and reduction. Similar prior arts are found with respect to a ceria porous material.
According to the above-mentioned pH swing method, which is substantially a method of producing alumina, the pH of boehmite (AlOOH), a precursor, is swung by use of an acidic material and an alkaline material to cause crystals to dissolve and to precipitate alternately thereby letting the crystals grow in a porous fibrous shape with a narrowpore size distribution. However, because the pH should be swung many times, the process is time-consuming and meets difficulty in controlling the conditions for product consistency. Further, when a second component is to be incorporated, it once settles but is then solubilized because of the pH variations, failing to be uniformly dispersed. Or, where a desired second component is such that forms a precipitate at a pH out of a range of from 6 to 11, it is impossible to incorporate the second component into the precursor. Furthermore, the conventional pH swing method does not provide an alumina porous material having a spongy structure nor a porous material comprising an aggregate of particles having an aspect ratio of 3 or smaller.
In particular, the porous materials described in JP-A-58-190823 and JP-A-60-54917 are composed of fibrous particles. When used as a catalyst carrier, a porous material comprising an aggregate of fibrous particles might be capable of supporting a noble metal in a high disperse state. However, as will be explained later in more detail, there will be a certain crystal plane along the fiber length direction so that the catalyst component tends to be supported on that plane in an increased proportion. This helps the catalyst component agglomerate in high temperature.
The spherical silica proposed in JP-B-7-64543 supra is composed of amorphous particles. Where used as a catalyst carrier, it provides no crystal plane to support a noble metal in a high disperse state. It follows that the noble metal particles easily move on the catalyst surface to undergo sintering, resulting in reduction of activity. Silica has lower affinity to noble metal than, for example, alumina, which also contributes to sintering of the supported noble metal particles and reduction of activity. Additionally, where the silica porous material is used in a three-way catalyst, coking occurs to deactivate the catalyst.
None of the aforementioned other prior arts relating to zirconia, titania, magnesia, iron oxide or ceria porous materials proposes a porous material having a spongy structure characterized in that (1) the mean pore diameter is in a meson-pore region, (2) the pores have a sharp size distribution, (3) at least a part of the pores have a three-dimensional network structure, and (4) the porous material has substantially no fibrous structure, or a porous material which is made up of an aggregate of particles having an aspect ratio of 3 or smaller and has the characteristics (1) to (4), still less a process for producing such porous materials.
On the other hand, a three-way catalyst has conventionally been used for treating auto exhaust gases, which catalyzes oxidation of CO and HC and reduction of NOx simultaneously. Conventional three-way catalysts widely known for this particular use comprise, for example, a heat-resistant base material made of, e.g. cordierite, a porous carrier layer of γ-alumina formed on the base material, and a noble metal catalyst component, such as platinum or rhodium, supported on the porous carrier layer. A three-way catalyst additionally containing ceria (cerium oxide) having oxygen storage ability to have increased low-temperature activity is also known (see JP-B-6-75675).
However, when these catalysts are exposed to high-temperature exhaust for a long time, the noble metal shows grain growth to reduce its catalytic activity on the simultaneous oxidation-reduction reactions of CO, H2, HC, and NOx in exhaust gases. This is considered to be one of the causes for three-way catalysts to reduce their high-temperature durability.
Carbon dioxide (CO2) in exhaust gases from internal combustion engines of automobiles and the like has now given rise to a serious problem to global environment conservation. A so-called lean-burn engine using a lean fuel mixture is a promising measures against this problem. Lean burn engines use less fuel thereby to suppress CO2 generation.
Since the conventional three-way catalysts aim at simultaneous oxidation of CO and HC and reduction of NOx in exhaust gases at a stoichiometric air/fuel ratio, they are inadequate to reduce NOx in the oxygen-excess atmosphere as in the exhaust gas from lean-burn engines. Therefore, it has been demanded to develop an air cleaning system using a catalyst capable of removing NOx even in an oxygen-excess atmosphere.
Along this line, the present inventors have previously proposed a catalyst for purifying exhaust gases comprising an alkaline earth metal and platinum supported on a porous carrier of alumina, etc. (JP-A-5-317652) and a catalyst for purifying exhaust gases comprising lanthanum and platinum supported on a porous carrier (JP-A-5-168860). In these catalysts the oxide of an alkaline earth metal or lanthanum serves as a NOx storage component under lean conditions, and the stored NOx react with reducing components such as HC, CO, and H2 generated under stoichiometric conditions or in the state of transition from stoichiometric conditions to fuel-rich conditions (at air/fuel ratios lower than the stoichiometric point). Accordingly, they exhibit excellent performance in NOx removal even under lean conditions.
However, an exhaust gas also contains sulfur oxides (SOx) resulting from combustion of sulfur (S) present in fuel. SOx are oxidized by metallic catalyst components under lean conditions and also react with steam to generate sulfite ions or sulfate ions. The sulfite or sulfate ions can react with the NOx storage component to convert the NOx storage component to its sulfite or sulfate. This phenomenon is called sulfur poisoning. Sulfur poisoning impairs the NOx storing activity of the NOx storage component, which seems to be one of the causes of reduction in NOx removal performance. Upon being heated in a reducing atmosphere, the sulfite or sulfate releases sulfur and returns to its active form. However, if the sulfite or sulfate grows in grains, the sulfur content is hardly released by heating in a reducing atmosphere, and the NOx storing activity is hardly restored.
The recent improvements on engine combustion have made it possible to run lean-burn engines up to a high load, which has further increased the demand for a catalyst for purifying exhaust gases which have higher NOx removal performance. That is, the situation has required that a catalyst for purifying exhaust gases should have high NOx removal performance even in a high-temperature exhaust gas and undergo no reduction in NOx removal performance even when exposed to a high-temperature exhaust gas for a long time (this property will hereinafter be sometimes referred to as high-temperature durability).
However, when a catalyst for purifying exhaust gases is exposed to high-temperature exhaust for a long time, the noble metal shows grain growth to reduce its catalytic activity on the oxidation-reduction reactions. This is considered to be one of the causes of reduction in high-temperature durability of the catalyst.